Imaging X-ray facilities, such as, for example, computed tomography systems, which are embodied to acquire projection images from the inside of a patient's body are well known from the prior art. For examination, the patient is usually positioned on a patient couch, which is typically moved, in particular in the horizontal direction, during the examination. Herein, a region of the patient to be mapped is introduced into a tunnel-shaped examination region around which at least the X-ray source is rotated during the image acquisition. Known from the prior art are in particular computed tomography systems with stationary X-ray detectors or X-ray detectors that move in synchronism with the X-ray source. In the latter case, the X-ray detector, in particular a multi-line detector, and the X-ray source are arranged diametrically opposite the center of rotation.
In computed tomography, image acquisition is usually performed such that, at least for a part of the X-rayed body region, a complete set of projection images acquired from different directions is available. A complete set of projection images enables a three-dimensional reconstruction via filtered back projection of the mapped region. In particular in spiral computed tomography, the patient couch or the patient table is continuously pushed in the horizontal direction while the X-ray source rotates. The feed in units of the effective detector width in the center of rotation with each rotation is generally called the pitch or pitch factor.
The recording parameters, in particular the pitch factor, limit the size of the field of view containing a complete set of projection images for reconstruction. Furthermore, in particular iterative methods are known that enable reconstruction in fields of view which do not contain complete datasets. However, these methods have drawbacks with respect to the achievable image quality and accuracy of the image values. Moreover, artifacts occur to a greater degree at the transition to the field of view that contains complete projection data.
This is particularly disadvantageous when the projection images or a three-dimensional volume reconstructed from the projection images or image slices of the reconstructed three-dimensional volume are to be used as the basis for planning and/or optimization of subsequent radiotherapy. In this case it is essential that it is possible for organs, in particular high-risk organs, located in the field of view to be acquired, to be segmented or contoured exactly.
U.S. Pat. No. 9,402,587 B2 discloses a method for recording projections during a spiral scan. U.S. Pat. No. 9,247,912 B2 discloses a helix scan method with a pitch of at least three.
U.S. Pat. No. 7,978,810 B2 discloses an imaging method for variable pitch spiral CT.